The present invention is directed, in general, to the reception of high definition television signals and, more specifically, to a system and method for recovering a recurring data segment synchronization pattern in the presence of an arbitrary phase rotation of the pilot carrier by detecting and compensating for the amount of the phase rotation.
The Advanced Television Systems Committee (ATSC) standard for digital high definition television (HDTV) transmission uses a signal that is modulated as an eight (8) level vestigial sideband (VSB) symbol stream with a rate of 10.76 MHz. The ATSC standard defines a data segment as a length of data having eight hundred thirty two (832) symbols. Each data segment begins with a fixed pattern comprising the four symbols xe2x80x9c1001xe2x80x9d. The fixed pattern of xe2x80x9c1001xe2x80x9d indicates the start of a new data segment. The xe2x80x9c1001xe2x80x9d symbols are the first four symbols of the 832 symbols in the data segment.
In the demodulation process, the fixed pattern xe2x80x9c1001xe2x80x9d is used by the receiver to determine the positional alignment of the data segment. Locating (or xe2x80x9crecoveringxe2x80x9d) the positional alignment of the data segment is essential for data frame synchronization and for facilitating symbol timing recovery. Consequently, reliable and robust detection of the recurring four symbol xe2x80x9c1001xe2x80x9d pattern is essential to the operation of any receiver that receives signals that are broadcast according to the ATSC standard.
The baseband signal to be demodulated includes a pilot carrier located exactly at DC on the frequency spectrum. Under normal conditions, the pilot carrier is at the same phase angle (or very near the same phase angle) as the remainder of the signal spectrum. Under these normal conditions, any of a number of well known techniques may be used to recover the data segment synchronization pattern.
If, however, the channel introduces severe phase distortion over the signal spectrum so that the pilot carrier is at a very different phase angle from the remainder of the signal spectrum, then the recovery of the data segment synchronization pattern using standard methods will become much more difficult or even impossible. Severe phase distortions can arise, for example, in a channel where there is a very strong multipath with a very short delay on the order of one symbol period or less.
Under normal conditions in a terrestrial data transmission system, a conventional channel equalizer is used to compensate for channel distortion and multipath. Under conditions of severe phase distortion, a complex channel equalizer operated in a xe2x80x9cblindxe2x80x9d adaptation mode can be used to compensate for channel effects before the data segment synchronization recovery is performed. Although the length of the complex channel equalizer is not very long (on the order of ten (10) to twenty (20) data taps), the complex channel equalizer nevertheless constitutes a substantial hardware component of a demodulator system.
Because a complex channel equalizer performs a two-dimensional complex equalization (as opposed to a conventional equalizer that performs real, or one-dimensional, equalization), a complex channel equalizer can become susceptible to instability in the presence of certain nonlinearities in a complex input signal. Under such conditions, recovery of a data segment synchronization pattern using a complex channel equalizer can be very difficult or even impossible.
There is therefore a need in the art for an improved system and method of recovering a recurring data segment synchronization pattern in the presence of an arbitrary phase rotation of a pilot carrier in an ATSC broadcast signal. There is also a need in the art for an improved system and method that can recover a recurring data segment synchronization pattern without using a complex channel equalizer. There is also a need in the art for an improved system and method that can compensate a baseband signal for an arbitrary phase rotation of a pilot carrier.
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide an improved system and method of recovering a recurring data segment synchronization pattern in the presence of an arbitrary phase rotation of a pilot carrier in a terrestrial HDTV broadcast signal encoded according to the ATSC standard.
Under specified channel conditions, the present invention allows the data segment synchronization pattern to be detected before the signal is compensated for the phase rotation of the pilot carrier. The present invention compensates for the phase rotation of the pilot carrier prior to the channel equalization and subsequent forward error correction of the signal.
The present invention comprises a first synchronization pattern detector for detecting a data segment synchronization pattern on the real component of the signal and a second synchronization pattern detector for detecting a data segment synchronization pattern on the imaginary component of the signal. This arrangement permits the present invention to detect the data segment synchronization pattern on a signal whose pilot carrier has been rotated by an arbitrary phase angle.
The present invention also comprises circuitry for compensating for the phase rotation of the pilot carrier. The present invention also comprises circuitry for compensating for distortions in signal gain that may occur when pilot carrier experiences phase rotation.
It is a primary object of the present invention to provide an improved system and method for recovering a recurring data segment synchronization pattern in the presence of an arbitrary phase rotation of a pilot carrier.
It is an additional object of the present invention to provide an improved system and method that can recover a recurring data segment synchronization pattern without using a complex channel equalizer.
It is another object of the present invention to provide an improved system and method for compensating for an arbitrary phase rotation of a pilot carrier.
It is still another object of the present invention to provide an improved system and method for compensating for signal gain in a signal in which pilot carrier phase rotation has occurred.
It is yet another object of the present invention to provide an improved feedforward system and method for compensating for pilot carrier phase rotation and signal gain.
It is also another object of the present invention to provide an improved feedback system and method for compensating for pilot carrier phase rotation and signal gain.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the DETAILED DESCRIPTION, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprise,xe2x80x9d as well as derivatives thereof, mean inclusion without limitation; the term xe2x80x9cor,xe2x80x9d is inclusive, meaning and/or; the phrases xe2x80x9cassociated withxe2x80x9d and xe2x80x9cassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term xe2x80x9ccontrollerxe2x80x9d means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.